Strange artifacts during camera transition when rendering movie
It seems like the motion wasn't estimated correctly. As a workaround you could try to set the GOP Size (Keyframe interval) so that a keyframe is set when the movement starts.

This seems like an encoder/Blender bug. If such errors occurred at multiple times you could encode with a lossless video format and use one of the many third party video converters as a second step. Possibly with multipass encoding enabled.